I said it once I'll say it again.
"From the second you turned on the game, you were already in Eternum."
That's my theory. There's no way the main character can trigger important events just from starting the game. As if quests and mysteries were all on hold until he got there. The hints are all there. "That never happened before. I never saw her before." Why would NPCs suddenly step forward and give more hints on a quest that was never solved before then? But now it suddenly has more clues when the MC shows up? It's like they said in Disgaea 2, "maybe the only reason you weren't turned into a monster is because you're the main character."
The main character is playing an offline game. All the girls in it that he loves are just NPCs. "They remember you. It's hard to tell who's a player and an NPC." And quests activate and progress when he's around because he's a player and the ONLY player. "(voice like All-Might) You can move on in a quest, BECAUSE I AM HERE!" He just forgot that he wipes his memory, and that he is playing a game when he starts (which he did for immersion, like in that Dragon Quest animated movie). " Hey, bored with playing games and always winning? You want full immersion and you're the founder or veteran player of a game? Memory wipe and play with no weapons and and only your allies are OK at the game, but no one is OP."
MARK MY WORDS!
At the end of the game I bet the MC is alone and forced to accept that he got so into immersion of Eternum that he fell in love, not just with the game, but the NPC love interests he made in it as well. And so he deletes the game and decides to go have real relationships again.